Arrangements for controlling the rotational speed of internal combustion engines are known; thus, for example, it is customary to provide an idle air charge control (IACC) adapted to act upon an air bypass arranged parallel to the throttle valve of the internal combustion engine. The action upon the air bypass conventionally is done with the aid of a two-winding rotational controller. Considering the instantaneous actual speed of the internal combustion engine, a desired speed and other peripheral data, such an idle air charge control (when executed pursuant to analog or digital technology) is under most operating conditions in a position to maintain a perfect idling condition of the internal combustion engine. Problems occur in the transition phase from the overrun mode of operation to idling or from the driving mode of operation to idling.
On the other hand, it is known in the operation of internal combustion engines to interrupt the supply of fuel if the throttle valve is closed at higher and high engine speeds, that is, if the internal combustion engine is in the overrun mode of operation. Such an overrun condition exists also if the rotational speed of an internal combustion engine is higher than corresponds to the position of the throttle valve in a spark-ignition engine or to the quantity of fuel injected in a diesel engine, for example. If the internal combustion engine is in the overrun mode of operation, no output is desired. Therefore, fuel delivery to the internal combustion engine through the relevant fuel supply apparatus (carburetor, injection systems, or the like) is conventionally reduced or interrupted entirely (overrun cutoff OC).
If an internal combustion engine runs at a rotational speed above idling speed, it can be of decisive importance that the system controlling the operating condition of the internal combustion as a whole, that is, fuel supply apparatus such as carburetor, fuel injection system, or the like, as well as other peripheral subsystems, also for ignition systems, and the like, is provided with information as to whether this rotational speed is attributable to overrunning.
At rotational speeds above idling, it may even happen that an overrun cutoff function is activated as an independent subsystem for idle air charge control, resulting in an undesired shutoff of the fuel supply, so that in the extreme case the engine may even stall. Critical operating conditions in the overrun mode of operation occur when the clutch is disengaged or when a vehicle equipped with automatic transmission is decelerated because in these cases the internal combustion engine is no longer entrained by the vehicle moved along or driven by other means (downhill driving). In such a case, the speed may drop so abruptly that the idle air charge control is unable to intercept it in time.
Whether or not the internal combustion engine is in the overrun mode of operation cannot be determined by the rotational speed and throttle flap position sensors alone. However, a distinction is desired, for example, to initiate appropriate measures on detection of an overrun condition to prevent the internal combustion engine from stalling or the overrun cutoff functions from being enabled in the presence of an increased idling speed which is attributable to the idle air charge control.